diff --git a/tools/prepare-package.js b/tools/prepare-package.js
index 6d9750d709..a1c250a834 100644
--- a/tools/prepare-package.js
+++ b/tools/prepare-package.js
@@ -8,33 +8,37 @@ const { cleanSourceMapRoot } = require('../.make-helpers');
 const ROOT = 'dist/';
 const CJS_ROOT = ROOT + 'cjs/';
 const ESM5_ROOT = ROOT + 'esm5/';
-const ESM2015_ROOT = ROOT + 'esm2015/';
+const ESM_ROOT = ROOT + 'esm/';
 const UMD_ROOT = ROOT + 'global/';
 const ESM5_FOR_ROLLUP_ROOT = ROOT + 'esm5_for_rollup/';
 const TYPE_ROOT = ROOT + 'types/';
 const UMD_PKG = ROOT + 'bundles/';
 
 // License info for minified files
-let licenseUrl = 'https://github.com/ReactiveX/RxJS/blob/master/LICENSE.txt';
-let license = 'Apache License 2.0 ' + licenseUrl;
+const licenseUrl = 'https://github.com/ReactiveX/RxJS/blob/master/LICENSE.txt';
+const license = 'Apache License 2.0 ' + licenseUrl;
 
 // Execute build optimizer transforms on ESM5 files
 klawSync(ESM5_ROOT, {
   nodir: true,
   filter: function (item) {
     return item.path.endsWith('.js');
-  }
+  },
 })
-  .map(item => item.path.slice((`${__dirname}/${ESM5_ROOT}`).length))
-  .map(fileName => {
-    if (!bo) return fileName;
-    let fullPath = path.resolve(__dirname, ESM5_ROOT, fileName);
+  .map((item) => item.path.slice(`${__dirname}/${ESM5_ROOT}`.length))
+  .map((fileName) => {
+    if (!bo) {
+      return fileName;
+    }
+    const fullPath = path.resolve(__dirname, ESM5_ROOT, fileName);
     // The file won't exist when running build_test as we don't create the ESM5 sources
-    if (!fs.existsSync(fullPath)) return fileName;
-    let content = fs.readFileSync(fullPath).toString();
-    let transformed = bo.transformJavascript({
+    if (!fs.existsSync(fullPath)) {
+      return fileName;
+    }
+    const content = fs.readFileSync(fullPath).toString();
+    const transformed = bo.transformJavascript({
       content: content,
-      getTransforms: [bo.getPrefixClassesTransformer, bo.getPrefixFunctionsTransformer, bo.getFoldFileTransformer]
+      getTransforms: [bo.getPrefixClassesTransformer, bo.getPrefixFunctionsTransformer, bo.getFoldFileTransformer],
     });
     fs.writeFileSync(fullPath, transformed.content);
     return fileName;
@@ -44,10 +48,10 @@ if (fs.existsSync(UMD_ROOT)) {
   fs.copySync(UMD_ROOT, UMD_PKG);
   // Clean up source map paths so they can be re-mapped
   klawSync(UMD_PKG, { filter: (item) => item.path.endsWith('.js.map') })
-    .map(f => f.path)
-    .forEach(fName => {
+    .map((f) => f.path)
+    .forEach((fName) => {
       const sourceMap = fs.readJsonSync(fName);
-      sourceMap.sources = sourceMap.sources.map(s => {
+      sourceMap.sources = sourceMap.sources.map((s) => {
         const nm = 'node_modules/';
         const rr = path.resolve(ESM5_FOR_ROLLUP_ROOT);
         if (s.includes(nm)) {
@@ -73,6 +77,12 @@ fs.removeSync(CJS_ROOT + '/internal/umd.js');
 fs.removeSync(CJS_ROOT + '/internal/umd.js.map');
 fs.removeSync(ESM5_ROOT + '/internal/umd.js');
 fs.removeSync(ESM5_ROOT + '/internal/umd.js.map');
-fs.removeSync(ESM2015_ROOT + '/internal/umd.js');
-fs.removeSync(ESM2015_ROOT + '/internal/umd.js.map');
+fs.removeSync(ESM_ROOT + '/internal/umd.js');
+fs.removeSync(ESM_ROOT + '/internal/umd.js.map');
 fs.removeSync(TYPE_ROOT + '/internal/umd.d.ts');
+
+// Create `package.json` files for the ESM5 and ESM2015 roots that
+// instruct NodeJS to treat `.js` files inside as ESM.
+const esmPkgJson = JSON.stringify({ type: 'module', sideEffects: false });
+fs.writeFileSync(path.join(ESM5_ROOT, 'package.json'), esmPkgJson);
+fs.writeFileSync(path.join(ESM_ROOT, 'package.json'), esmPkgJson);